1. Field of the Invention
The present invention relates to a method of manufacturing a laminated piezoelectric-electrostrictive actuator (hereinafter called piezoelectric actuator or simply actuator) or more particularly to a method of mass-producing a laminated piezoelectric actuator.
As mechatronics which applies electronics to machinery is developing in recent years, more and more attention is being paid to actuator technology. An actuator is a device to perform a process control by use of pneumatic, hydraulic or electronic signals; for example, a driver for driving a print head of a dot printer.
Most actuators are activated by use of electromagnetic power. For instance, a dot printer uses an electromagnet which is constructed by winding an excitation coil on a magnet base of a soft ferromagnetic material as an actuator.
The print head, which has a printing wire provided on its point, is driven such that the print head is attracted to and released from the electromagnet by a signal current flowing through the excitation coil. Printing is performed when the print head projects the printing wire. However, it is a problem that an actuator using electromagnetic power is power-consuming and generates heat and electromagnetic noise and, therefore, a new type of actuator free from this problem is desired.
Meanwhile, since an actuator utilizing piezoelectric effect of a strong dielectric substance is not only free from this problem but is small-sized and light-weight and also proof against vibration and impact, its practical use or a method of mass-producing a laminated piezoelectric actuator is in great demand.
2. Description of the Related Art
It is well-known that a type of ceramic shows the best characteristics for the actuator utilizing the piezoelectric effect.
The ceramics of ferroelectric material are made by firing lead-nickel niobate, lead titanate, and lead zirconate mixed in an appropriate molecular weight ratio. A laminated ceramic capacitor-type actuator is well-known as a piezoelectric actuator.
FIG. 1 is a sectional diagram showing a first type of actuator of the related art of the present invention, i.e., an actuator of a laminated ceramic capacitor type.
The laminated ceramic capacitor-type piezoelectric actuator is fabricated through the following processes:
A solvent, a binder and a plasticizer are added to the above-mentioned mixture of the three ceramics materials and the mixture is kneaded to produce a slurry. The slurry is deposited and finished to a pre-determined thickness using the `doctor blade method` and dried to produce a green sheet.
Conductive paste is screen-printed on the green sheet to form an internal electrode layer thereon in such a pattern that, when the green sheet is cut into a size of a unit actuator element in a later process, an internal electrode 3r reaches the very end on one side of and partway to the end on the opposite side (with a margin 6r left as shown in FIG. 1) of the actuator element.
A plurality of green sheets with the internal electrode layer thus-printed are superposed in layers such that, when they are cut into the actuator element in the later process, the margin 6r and the internal electrode 3r appear alternately on each side of the actuator element.
The superposed green sheets are pressurized to be integrated in laminated layer, heated at a low temperature to remove the binder, then heated at a high temperature to sinter the ceramic raw material powder into a ceramics 2r. The thus-formed laminated ceramic layers 2r with the internal electrode layers sandwiched in between (hereinafter called parent actuator) are cut into a plurality of actuator elements. Conductive paste is spread on each side of the actuator element where the internal electrodes 3r appear and fired to form an external electrode 4r. Thus, an actuator 5 as shown in FIG. 1 is completed.
With the actuator thus fabricated, only the area sandwiched between the upper and lower internal electrodes 3r acts as an active region and performs a piezoelectric operation. However, the area including the margin 6r, which becomes an inactive region since no voltage is applied thereto, does not perform a piezoelectric operation. Eventually, a problem has become clear that the boundary area between the active and inactive regions is easily destroyed, as a result of a life test which drives continuous pulses to the actuator.
To solve the problem, an overall electrode-type actuator was proposed.
FIG. 2 is a sectional diagram showing second type of actuator of the related art of the present invention, i.e., an actuator of an overall electrode type.
The overall electrode-type actuator is fabricated through the following processes:
A plurality of the above-mentioned green sheets are each printed with an internal electrode layers over the whole surface, superposed in layers and thereafter, made into actuator elements through the same processes as in the above example. Portions of the internal electrodes 3s which appear on the sides of the actuator element are covered with insulators 7s alternately on each side, by applying an insulating material such as glass paste thereon.
Finally, conductive paste is spread on both sides of the actuator element, over the insulators 7s and then fired to form the external electrodes 4s. The actuator thus fabricated has an extended life time because it does not have an inactive region.
However, a problem is that insulation coating work requires much intricate labor, and therefore the ceramics 2s or green sheet 1 cannot be too thin, considering the ease of work.
FIGS. 3A-3C are diagrams showing third type of laminated piezoelectric actuator of the related art of the present invention.
The actuator of this type is fabricated through the following processes:
First, conductive paste is applied over the entire surface of the above-mentioned green sheet 1, to form an internal electrode layer 8t. The green sheet 1 is covered with another green sheet 1, with the internal electrode layer 8t sandwiched and they are pressurized to form a bonded sheet 11 as shown in FIG. 3A.
Next, by using an automatic punch, a plurality of slits 5t having a length l and a width d along Y and X axes, respectively are bored through the bonded sheet 11 at an interval 2L (twice the spacing L of an actuator element) in a lines, with the slits 5t in adjacent lines staggered by the spacing L, as shown in FIG. 3B.
Then, insulator paste (e.g., glass-ceramics) is stuffed into the slits 5t of the bonded sheet 11 to form an insulator 7t therein. A plurality of the bonded sheets 11 are superposed in layers, with the slits 5t of adjacent (upper and/or lower) layers staggered in parallel, by the length L, as shown in FIG. 3C. The superposed bonded sheets 11 are pressurized to be integrated and fired as described above to produce a parent actuator which has ceramics layers laminated with internal electrode layers sandwiched therebetween and insulators 7t stuffed in the slits 5t and which is an aggregation of actuator elements.
Cutting the parent actuator along dotted lines 3X, 3Y as shown in FIGS. 3B and 3C (FIG. 3C is a sectional view taken along line M-N of FIG. 3B) produces actuator elements 30 with internal electrodes 3t and insulators 7t appearing alternately on the opposite sides of the actuator element 30. When an external electrode 4t is provided by screen-printing and baking silver (Ag) paste, for example, on both sides of the actuator element, an actuator is completed. However, although the thus-fabricated actuator does not have an inactive region, the manufacturing process requires such intricate labor as producing the bonded sheets 11 and superposing a few tens to a few hundreds of them exactly in layers, with vertically adjacent layers staggered by a predetermined spacing L, and therefore it is difficult to mass produce in this example, too.
In summary, it is a problem with the related art that the actuator has a short lifetime and/or the manufacturing process is not suited for mass production.